On Tuesday, as he swept all five Republican presidential primaries — the first held since his last credible opponent conceded the race — Mitt Romney declared "the start of a new campaign."

On Wednesday, Democrats engaged that campaign, as a superPAC that supports President Obama and an environmental group launched $1 million in new ads in the swing states of Colorado and Nevada that stress a connection between Romney and big oil, The Associated Press reports:

"The ad by Priorities USA Action and the League of Conservation Voters Victory Fund casts the likely Republican presidential nominee as the '$200 million man,' accusing Romney of supporting oil industry profits and tax breaks. It asserts that Romney is 'in the tank for big oil.'

"Priorities USA Action is a superPAC founded by two former Obama White House aides. It has struggled in fundraising compared with Republican-leaning superPACs like American Crossroads and its nonprofit arm, Crossroads GPS, which have raised $100 million this election cycle."

The contests were the first since former Pennsylvania Sen. Rick Santorum left the race on April 10.

As Politico's Morning Score noted Wednesday, Eric Ostermeier at the University of Minnesota's Humphrey School of Public Affairs' Smart Politics project added a note of caution for Romney, writing:

"A Smart Politics review of Republican primary election data since 1972 finds that Mitt Romney's performances in Delaware and Pennsylvania mark the first time a GOP front-runner has failed to reach the 60 percent mark in a contest conducted after his last major challenger dropped out of the race."

But Romney neared the 60 percent mark in both states — and exceeded it in the other three — even though Santorum, former House Speaker Newt Gingrich and Texas Rep. Ron Paul were on all five ballots.

Gingrich campaigned aggressively in Delaware, and finished second with 27 percent of the vote. Romney still got more than 56 percent of the vote.

And Pennsylvania is Santorum's home state, where he finished second with 18 percent of the vote. Romney still secured 58 percent of the vote in that battleground state.